The present invention relates to an axle suspension for a motor vehicle.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
The use of leaf springs is generally known for application in axle suspensions of motor vehicles. The leaf springs are either arranged as transversal leaf springs transversely to the travel direction of the motor vehicle or as longitudinal leaf springs in travel direction of the motor vehicle. Coupled to the leaf springs are axle components for attachment of the wheels. When transversal leaf springs are involved, the axle components can be constructed as multilink axles or wishbone axles and kinematically coupled with an axle auxiliary frame or the vehicle body. When driving over obstacles that cause vertical movement of the wheel, the wheel is guided via the kinematic coupling and jounced or rebound via the spring force of the transversal leaf spring.
Longitudinal leaf springs assume sometimes also guiding tasks, i.e. the kinematic coupling with an axle auxiliary frame or the body in addition to the suspension tasks. Longitudinal leaf springs find oftentimes application in combination with rigid axles in which the axle is coupled on two opposing transversal leaf springs. As a result, the longitudinal leaf springs absorb forces in x, y, and z directions of the motor vehicle.
Leaf springs have been made of steel material and more recently of lightweight material for use in motor vehicles. Examples include plastics or fiber composites which have suspension characteristics that resemble those of leaf springs of steel while having considerably lighter specific weight.
Longitudinal leaf springs are installed in the motor vehicle when unbiased or at rest. When acted upon by the static wheel load, the leaf spring is tensioned. In their initial state, longitudinal leaf springs have an arched configuration, with the arched profile pointing towards the roadway in the installation position. Once installed, the longitudinal leaf springs essentially assume a straight configuration in response to the static wheel load and accompanying spring compression. When exposed to dynamic wheel forces, jouncing and rebounding cause the longitudinal leaf springs to alternate between the arched configuration, when rebounding, and the straight configuration, when jouncing.
A particular problem is encountered during braking when the leaf spring undergoes a so-called S-deflection in response to the brake moment transmitted via the axle. The leaf spring deforms hereby as a response to the brake moment in the shape of an S so that a part, called arm, of the spring already under tension as a result of the static wheel load is even further tensioned while the other part of the spring is relaxed. This is illustrated by way of example in FIGS. 1a to 1c which show simplified side views of a conventional leaf spring 1. FIG. 1a shows hereby the leaf spring 1 in its initial state when the leaf spring is at rest and not under stress and has an arched profile. When the leaf spring 1 is installed in a motor vehicle and exposed to the static wheel load, the arched profile of the leaf spring 1 is changed to a substantially linear configuration when an inner stress is present. This is shown in FIG. 1b. When now encountering a brake action, the leaf spring 1 undergoes a deformation as shown in FIG. 1c. The broken line in FIG. 1c depicts the initial state of the leaf spring (FIG. 1a). As the leaf spring 1 is acted upon by a braking moment MB, a forward region 2 of the leaf spring 1 remains substantially free of stress whereas the leaf spring 1 is under much greater tension in a rearward region 3 and undergoes a significant deflection. The configuration of the leaf spring 1 resembles in longitudinal direction the shape of an S. For that reason, the braking action is labeled as S-deflection.
Exceeding a critical stress state within the spring may cause permanent damage and shorten the service life of the spring as a result of a fatigue behavior during frequent stress situations.
Leaf springs of fiber composite are oftentimes configured as components with many layers. Leaf springs encounter shear stress when the stress situation changes, causing softening of the leaf spring between the individual layers. As a result, individual fiber strands may tear or fracture or become so weakened in response to the fatigue behavior of the spring that service life is shortened.
It would therefore be desirable and advantageous to provide an improved leaf spring to obviate prior art shortcomings and to have a spring stiffness which can be maintained lastingly and suited to a stress situation during braking.